1. Field of the Invention
The present invention relates to a liquid crystal display device.
2. Description of the Related Art
Liquid crystal display devices have widely been disseminated as monitor devices of PCs (personal computers) from the viewpoints of their thinness, light-weight and low power consumption, wherein recent expansion of a market of digital television set has raised an increasing demand on TV-use liquid crystal panel capable of realizing high resolution, for which display quality comparable to that of CRT is required. In particular, liquid crystal display device is known to be slow in response speed when compared with CRT, so that it has been understood as an urgent mission to improve the response speed and to realize excellent movie characteristics.
One possible reason for such low response speed of the liquid crystal display device is that the liquid crystal molecule per se has only a slow response, and cannot make a response within a single frame under low temperatures or low gradation state, and this consequently raises a problem of causing blurring and afterimage in the movie. The liquid crystal display device uses light from an illumination device placed on the back side for the display, wherein the illumination device is kept active throughout one frame, and is therefore known to be inferior in the movie characteristics as compared with CRT and plasma display device which adopt pulse illumination within one frame. The former is referred to as “hold-type display”, and the latter is referred to as “impulse-type display”. The hold-type display is described in Non-Patent Document 1 listed later.
One of the techniques of improving the response speed of the liquid crystal display device per se is the overdrive technique shown in FIG. 6, which has widely been known. In the drawings of the normal operation 801 and the overdrive operation 802, the upper halves show response waveforms of luminance, and the lower halves show data waveforms. In the normal operation 801, an effective voltage 803 expresses an effective voltage of the data waveform, wherein response time T2 represents a response time of luminance within a single frame FR1 (duration of time having a luminance ratio of 10% to 90% kept therein). In the overdrive operation 802, the effective voltage expressed by the data waveform is increased by an increment 804 from that in the normal operation 801, showing response time T3 of luminance within a single frame FR1 shorter than response time T2.
Liquid crystal generally shows better responsiveness as the voltage applied thereto grows larger. The overdrive operation 802 is a technique of applying, at the rise-up time in the response, a voltage larger than the data voltage to intrinsically be applied so as to accelerate the response of liquid crystal, to thereby improve the response speed at gradation of low response speed. On the contrary, at the fall-down time in the response, the response is accelerated by applying a voltage lower than the intrinsic data voltage.
There are known methods of determining the increment (correction value) 804 of the effective voltage, including a method of determining the correction value for the m-th frame by comparing data between the m-th frame and the (m−1)-th frame; and a method of determining a correction value for the (m−1)-th frame by comparing data among the (m−2)-th frame, the (m−1)-th frame and the m-th frame.
The conventional operation may certainly improve the response speed by applying a voltage larger than the intrinsic data voltage in the period of the first frame (1/operation frequency), but the improvement is achievable only to as short as 16 ms or below, which corresponds to a period of a single frame under a 60-Hz operation, for a gradation range allowing only a low response speed of the liquid crystal per se. This is because too high voltage for accelerating the response may result in overshooting which adversely affect the movie display, so that the voltage value is limitative.
For VA-mode liquid crystal panel, a phenomenon has been confirmed in that disturbance in orientation of the liquid crystal molecules becomes serious under high voltage application. In the VA-mode as shown in FIG. 7, liquid crystal molecules 901 vertically aligned under no applied voltage (in the black level) begin to incline with increase in the applied voltage, as being affected by any structures provided in the panel or by the direction of electric field. In general, a maximum applied voltage gives the white level display, wherein the liquid crystal molecules 902, 903 incline to the largest degree. The liquid crystal molecules 902 express those in the while level display under normal alignment, having a liquid crystal alignment direction 912. The liquid crystal molecules 903 express those in the white level display under abnormal alignment, having a liquid crystal alignment direction 913. It is preferable in the white level display that the liquid crystal molecules incline in the normal direction of alignment, but the inclination under an abrupt voltage application may sometimes result in variation in the direction of alignment.
FIG. 8 is a drawing showing one-frame overdrive operation 1001 and two-frame overdrive operation 1002. In the one-frame overdrive operation 1001, the effective voltage expressed by the data waveform is increased by an increment 1007 as a result of the overdrive only in the first frame FR1. In the two-frame overdrive operation 1002, the effective voltage expressed by the data waveform is increased by an increment 1005 in the first frame FR1, and an absolute value of the effective voltage expressed by the data waveform is increased by an increment 1006 in the second frame FR2. The liquid crystal molecules 902 under the normal alignment shown in FIG. 7 successfully increase the luminance 1004 by the overdrive in the second frame FR2. Whereas, the liquid crystal molecules 903 under the abnormal alignment shown in FIG. 7 undesirably degrade the luminance 1003 in the second frame FR2 due to disturbance in the alignment of liquid crystal.
The liquid crystal molecules under the normal alignment contribute to the luminance to a maximum degree, whereas any molecules departing from such alignment are causative of lowering in the luminance. Also the molecules out of alignment may gradually recover the normal alignment with elapse of time under orientation-limiting force inside the panel, but the lowering in luminance may adversely affect the response waveform in the second frame FR2, or in other words, adversely affect the movie characteristics. Conversion of the data voltage in the second frame FR2 has therefore been necessary.
In this case, it takes two frames (32 ms) to reach the intrinsic data voltage level, and this has been one cause for degrading the movie characteristics.
Another known problem in the liquid crystal display device particularly designed for TV sets relates to that the VA-mode liquid crystal display device causes difference in the luminance and the chromaticity between display in the front view and display in oblique views. More specifically, the display device of this type looks whitish in oblique views due to an excessive luminance, showing a large difference in the chromaticity from the front view, and was therefore inappropriate as a liquid crystal display device possibly viewed also from oblique directions. Such nonconformity is ascribable to that the display by the liquid crystal display device makes use of birefringence, and that the VA-mode device causes difference in the gradation-luminance characteristics between oblique views and the front view, as the liquid crystal molecules incline to a larger degree.
FIG. 9A is a drawing showing the liquid crystal molecules as viewed from this side. Liquid crystal molecules 401 incline towards this side, and liquid crystal molecules 402 incline towards the far side. FIG. 9B is a drawing showing relations between applied voltage (gradation) and transmittance (luminance). A characteristic curve 411 expresses the characteristics observed in the front view, a characteristic curve 412 at 60° on the far side, a characteristic curve 413 at 60° upper in the quadrisection, and a characteristic curve 414 at 60° on this side.
The liquid crystal molecules 401 inclined towards this side and the liquid crystal molecules 402 inclined towards the far side differ from each other in the resultant gradation-luminance characteristics in oblique views, and the sum of the both gives an apparent gradation-luminance characteristic, and this raises differences in the luminance and the chromaticity in the oblique views.
Several methods have already been proposed in view of improving the viewing-angle-dependent gradation characteristics. Principles of these methods are based on provision of a plurality of gradation-luminance characteristics on a time scale so as to moderate the waviness in the gradation-luminance characteristics in the oblique views, and examples of which include a technique of providing a plurality of regions within a single pixel so as to allow different levels of voltage to be applied thereto, to thereby impart a plurality of gradation-luminance characteristics to the pixel; and a technique of applying different levels of voltage on the frame basis, so as to impart a plurality of voltage-luminance characteristics to each frame, to thereby achieve a desired luminance on the time-average basis. These are referred to as half-tone techniques. The techniques or imparting a plurality of gradation-luminance characteristics to a single pixel are typically described in Patent Document 1 listed later.
Watching TV also on the general PC monitors has been becoming more common in these days, and the situation demands improvement in the movie characteristics while also improving the viewing angle characteristics in the VA mode. There is, however, no known technique realizing the both at the same time, and adoption of two or more techniques in combination has undesirably resulted in lowering in the yield ratio and increase in the cost. Now is the time for individual manufacturers to phase into mass production, and the increase in the cost is a critical issue to be overcome.
Patent Document 2 listed below describes a display device capable of displaying an image using a plurality of frames within one second, wherein one frame F0 is displayed as being divided into at least two fields F1, F2, and a desired image is displayed at least in one sub-field 1F in the field F1 with a first luminance Tx, and an image substantially same as that displayed with the first luminance is displayed in the residual one sub-field 2F with a second luminance smaller than the first luminance but larger than 0.
Patent Document 3 describes that one frame is divided into two fields so as to achieve a double-speed operation, while driving a first field using corrected display data calculated from display data after being corrected by a predetermined conversion method, and driving a second field using the display data without such data conversion.
Related arts are disclosed in:
[Patent Document 1] Translated National Publication of Patent Application No. Hei 08-507880;
[Patent Document 2] Japanese Patent Application Laid-open No. 2000-338464;
[Patent Document 3] Japanese Patent Application Laid-open No. 2002-132224; and
[Non-Patent Document 1] The Institute of Electronics, Information and Communication Engineers, Technical Report EID2000-47 p 13-18 (2000-09).